Such power supply systems are used, for example, for electrical current leadthroughs in pinch seals, made from quartz glass or hard glass, of halogen incandescent lamps, discharge lamps or the like.
Since the glass of the pinch seal has a substantially lower coefficient of thermal expansion than the power supply lines provided for supplying electrical power to a luminous means arranged within the lamp vessel, it is not possible to fuse the power supply lines directly in the glass of the pinch seal. The mechanical stresses resulting from the different coefficients of thermal expansion would lead to cracks and ultimately to premature failure of the lamp. For this reason, thin molybdenum foils having sufficient ductility are often used, and these molybdenum foils make gas-tight electrical power supply possible despite the different coefficients of thermal expansion of the glass and the molybdenum. With such solutions, for example that known from U.S. Pat. No. 6,075,318, in each case the two opposing ends of the molybdenum foils are welded to an inner and an outer power supply wire made from molybdenum, and the resultant power supply system is positioned in the lamp vessel end such that the inner power supply lines protrude into the interior of the lamp vessel, and the outer power supply lines protrude out of said lamp vessel. Subsequently, the glass at the lamp vessel end is heated and is pinched, for example by means of pinching jaws, with the power supply system in a gas-tight manner to form a pinch seal. The molybdenum foil on the one hand is used to produce the electrically conductive connection between the luminous means arranged within the lamp vessel and the power supply lines and, on the other hand, ensures a gas-tight seal of the lamp vessel.
It has been shown that the molybdenum foils and power supply lines, in particular in the case of lamps having a high thermal load, tend towards the formation of molybdenum oxides, for example MoO2, MoO3, the oxides initially forming on the outer power supply lines and then progressing to the molybdenum foils. Owing to this temperature-dependent oxide formation, the volume of the mentioned components increases and causes a significant increase in stress in the pinch seal which may lead to breakage and thus to premature failure of the lamp.
In order to improve the oxidation resistance and thus to increase the thermal loading capacity of the power supply system, it is known from the general prior art to apply a partial chromium plating. In this very work-intensive method, the power supply systems usually produced by resistance welding are embedded in a sand-like substrate up to the desired height of the chromium plating, for example 2 mm above the weld joint. Owing to a chemical reaction, partial chromium deposition takes place in the embedded region in an environmentally harmful process. The chromium layer formed improves the oxidation resistance and thus leads to an increase in the thermal loading capacity of the lamps. Disadvantages in such power supply systems are firstly that, owing to an insufficient sealing effect in the chromium-plated region, the effective sealing length of the molybdenum foils is reduced and, as a result, the sealing effect of the pinch seal is impaired with reduced life of the lamp and, secondly, the coating process is not only environmentally harmful but is also complex and cost-intensive owing to the largely manual production process. One further disadvantage is the fact that the outer power supply lines can easily break out of the pinch seal owing to axial tensile forces.